... with certificate, coal mine used ,best quality1.AplicationKTH106-Z mining explosion-proof intrinsically safe automatic telephone is produced according to the standard ... audio frequency and impulse signal sending.5)with features of explosion-proof, anti-moisture, volume small, weight light, solidity, durability ... ±1dB4)dimension 5)weight:4.4KGS POPULAR!!! BOZZ Mining Explosion-proof Automatic Telephone